1. Field of the Invention
The present invention relates to a semiconductor device including a circuit constructed by thin film transistors (hereinafter referred to as TFTs) and a method of fabricating the same. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel, and an electronic equipment including such an electro-optical device as a part.
Incidentally, in the present specification, the term “semiconductor device” indicates any devices capable of functioning by using semiconductor characteristics, and any of the electro-optical device, semiconductor circuit, and electronic equipment is a semiconductor device.
2. Description of the Related Art
In recent years, attention has been paid to a technique for constructing a thin film transistor (TFT) using a semiconductor thin film (its thickness is about several to several hundred nm) formed on a substrate having an insulating surface. The thin film transistor is widely applied to an electronic device, such as an IC or an electro-optical device, and especially as a switching element of an image display device, its development has been hastened.
As a typical example of the electro-optical device, a liquid crystal display device, an EL display device, or a contact type image sensor can be cited.
In general, the liquid crystal display device includes a pair of substrates which are opposed to each other at a certain substrate interval, particulate spacers for keeping the certain substrate interval, and a liquid crystal material sealed between the substrates.
The substrate interval of the liquid crystal display device is normally set to 1 to 20 μm, and this must be uniformly controlled with accuracy of about ±0.1 μm. This is because if fluctuation occurs in the substrate interval, not only deterioration in display quality, such as generation of irregular color or interference fringe, is caused, but also trouble, such as circuit damage or disabled display, is caused by contact of electrodes when the substrate interval is narrowed by an external force. Like this, the spacer is an important member for maintaining the performance of the liquid crystal display element.
Hereinafter, a conventional method of fabricating a liquid crystal display device (TFT-LCD) will be described in brief.
First, a pair of substrates are prepared. TFT elements and pixel electrodes are formed in matrix form on one of the substrates. Electrodes, color filters or the like are formed on the other substrate. Next, after an alignment film is formed on each of the pair of substrates, a rubbing processing is performed.
Next, particulate spacers are uniformly sprayed on the alignment film of either one of the substrates. Next, the one substrate is combined with the other substrate, and their peripheral portions are sealed with an adhesive for sealing, so that a liquid crystal cell is formed. Next, after the liquid crystal cell is filled with a liquid crystal material by a vacuum injection method, an injection port is sealed.
The foregoing flow of steps is a general fabricating process of a TFT-LCD.
In the above conventional steps, it is difficult to uniformly spray the particulate spacers, and there have been problems that transmissivity is lowered by aggregation of the spacers, and an element just under the spacer is destroyed to generate a leak or short circuit.
Besides, in the step of injecting the liquid crystal material by the vacuum injection method, center portions of the substrates become recess-shaped at both surfaces by pressurization at the time of injection, and in this periphery, the conventional particulate spacer does not have sufficient compression strength and is destroyed, or the spacer is moved and the trace of the movement causes orientation defects.
In the case where the generally used conventional particulate spacers (glass beads, plastic beads, etc.) are used, there is adopted a method of spraying the particulate spacers onto one of substrates. Thus, the spacers are disposed on a pixel electrode, and block incident light or disturb the orientation of liquid crystal molecules. As a result, it has been difficult to adjust the transmitted light amount or coloring. Besides, the particulate spacers are easily charged with static electricity, so that the spacers become easily aggregate and are difficult to be uniformly distributed.